Method of controlling the direction of propagation of injection fractures in permeable formations

ABSTRACT

The invention relates to a method of controlling the production of oil or gas from a formation ( 1 ) comprising that a first and second drilled production well ( 105, 110 ) are formed next to each other that extend essentially horizontally, that, at the drilled production wells, a further drilled well ( 115 ) is formed that extends between the first and the second drilled production well ( 105, 110 ), that the production of oil or gas is initiated, and that, while oil or gas is being produced, a liquid is conveyed to said further drilled well ( 115 ) and out into the formation ( 1 ) for a first period of time T 1 . The invention is characterised in that the pore pressure of the formation is influenced during the period T 1  with the object of subsequently controlling the formation of fractures along a drilled well, typically across large distances in the reservoir. Such influence is accomplished partly by production in adjacent wells, partly by injection at low rate without fracturing in the well in which the fracture is to originate. Injection at low rate presupposes that an at least approximated determination is performed of the maximally allowable injection rate I max  for the period T 1  in order to avoid fracturing ruptures in said further drilled well ( 115 ) when liquid is supplied by the injection rate I for the liquid supplied to the further drilled well being kept below said maximally allowable injection rate I max  for said first period of time T 1  when the relation σ′ hole,min &lt;=σ′ h  has been complied with.

FIELD OF THE INVENTION

The present invention relates to an improved method of the general kindwherein, for the production of oil or gas from a formation, a first anda second drilled production well are formed next to each other, andwherein a further drilled well, a so-called injection well, isestablished that extends at and between the first and the second drilledwell, wherein—while oil or gas is being produced—a liquid is conveyed tothe drilled injection well and out into the formation for a period oftime T₁.

BACKGROUND

The invention is based on the fact that, during supply of liquid to adrilled injection well at high injection rates, fractures may occur thatpropagate from the drilled injection well through those areas of theformation that have inherent weaknesses and/or in the direction of themaximal horizontal stress σ′_(H) of the formation. These fractures areundesirable in case they mean that liquid flows away uncontrollably fromthe drilled injection well directly into either the first or the secondadjoining drilled production well, which would mean that the operatingconditions are not optimal. However, in general the formation offractures has the advantage that the supplied liquid can more quickly beconveyed into the surrounding formation across a larger vertical faceand is thus able to more rapidly displace the contents of oil or gas.

SUMMARY OF THE INVENTION

By the invention it is attempted to provide a very particular fracturethat extends from a drilled injection well in order to optimise theproduction of oil or gas. More specifically the present invention aimsto enable control of the propagation of such fracture in such a mannerthat the fracture has a controlled course and will to a wide extentextend in a vertical plane along with and coinciding with the drilledinjection well.

This is obtained by performing, in connection with the method describedabove, at least an approximated determination of the maximally allowableinjection rate I_(max) during the period T₁ to avoid fracturing in thedrilled injection well when liquid is supplied, in that the injectionrate I for the liquid supplied to the drilled injection well is keptbelow said maximally allowable injection rate I_(max) for said firstperiod of time T₁, and in that the injection rate I is increased to avalue above I_(max) following expiry of the period of time T₁ when therelation σ′_(hole,min)<=σ′_(h) has been complied with. The term‘injection rate’ as used herein in this context is intended to designatethe amount of liquid, expressed as amount per time unit, supplied to thedrilled injection well.

U.S. Pat. No. 5,482,116 teaches a method of controlling the direction ofa hydraulic fracture induced from a wellbore. The method does not makeuse of induced changes to the stress field by production and injectionbefore fracturing.

In the present invention, the maximally allowable injection rate I_(max)for avoiding fracturing may eg be determined or estimated by theso-called ‘step rate’ test, wherein the injection rate is increased insteps while simultaneously the pressure prevailing in the well bore ismonitored. When the curve that reflects this relation suddenly changesits slope, such change is—in accordance with current theories—construedas on-set of fracture, propagation, and the injection rate I thatproduces such fracture formation is, in the following, designatedI_(max).

It is preferred that the drilled wells are established so as to extendessentially horizontally, whereby the vertical stresses of the formationcontribute further to the invention. The term ‘essentially horizontally’as used in this context is intended to designate well bores that extendwithin an angle range of +/−about 25° relative to the horizontal plane.It is noted that the invention may also be practised outside this range.

It is further preferred that, prior to establishment of the well bores,the direction of the largest effective inherent principal stress σ′_(H)of the formation in the area of the planned location of the well boresis estimated, and that the drilled wells extend within the interval+/−about 25° relative to this direction.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

FIG. 1 shows two drilled production wells, from which oil or gas isproduced, and the orientation of the principal stresses in thesurrounding formation;

FIG. 2 shows the stresses in the formation shown in FIG. 1 following sixmonths of production,

FIG. 3 shows two drilled production wells, from which oil or gas isproduced, and a drilled injection well to which liquid is supplied, andthe orientation of the principal stresses in the surrounding formation;

FIG. 4 shows the stresses in the formation shown in FIG. 3 following sixmonths of production and three months of water injection;

FIG. 5A shows the principal stresses acting on a unit element around thedrilled injection well;

FIG. 5B diagrammatically shows the minimum state of stress around thewell;

FIG. 5C is a section taken on line A—A in FIG. 5B indicating minimumhole stress.

FIG. 6 shows the development, over time, of the stresses immediatelyabove the drilled injection well shown in FIG. 5; and

FIG. 7 illustrates a typical relation between the pressure in theinjection well and the injection rate.

DETAILED DESCRIPTION

In FIG. 1 reference numerals 5, 10 designate two drilled productionwells for the production of oil or gas from a Cretaceous formation 1.The drilled production wells 5, 10 extend in an approximately sharedplane in the formation 1 at a depth of eg about 7000 ft below sea level.The shown shared plane is horizontal, but it may have any orientation.For instance, the drilled production wells 5, 10 may extend in a planewith a slope comprised within the interval +/− about 25° relative to thehorizontal plane.

In a conventional manner the drilled production wells 5, 10 are, viaupwardly oriented well bores in the areas 16, 20, connected to a wellhead, from where oil or gas from the formation 1 is supplied to adistribution system on the surface. The well bores 5, 10, 16, 20 areestablished, as is usually the case, by drilling from the surface.

The drilled production wells 5, 10 may have a longitudinal expanse of egabout 10,000 ft and preferably extend mutually in parallel, eg at adistance of about 1200 ft. The drilled production wells 5, 10 may,however, within the scope of the invention, diverge slightly in adirection from the areas 16, 20. The situation shown in FIG. 1 isrepresentative of an authentically occurring course of drilling, thescale shown describing distances in ft.

The invention aims at providing, in the formation, a stress field thatensures that a fracture generated by injection at sufficiently elevatedpressure and rate extends along the well at which the fracture isinitiated

The invention presupposes knowledge of the initial state of stresses ofthe formation, ie the state of stresses prior to the up-start of anysubstantial production or injection. In many cases the stress field inthe formation will initially be oriented such that the principalstresses are constituted by two horizontal stress components and by onevertical stress component. In such cases, determination of the initialeffective stress field requires determination of four parameters: σ′_(v)that is the vertical effective stress component, σ′_(H) that is themaximal horizontal effective stress component, and σ′_(h) that is thehorizontal effective stress component perpendicular to σ′_(H), and thedirection of σ′_(H). The value of σ′_(V) is given by the weight of theoverlaying formation minus the pressure, p, of the pore fluid. Thepressure p of the pore fluid can be measured from the wall of a drilledwell by means of standard equipment. The weight of the overlayingformation can be determined eg by drilling through it, calculating thedensity of the formation along the drilled well on the basis ofmeasurements taken along the drilled well, and finally determining thetotal weight per area unit by summation. In cases when σ′_(V) is thelarger of the three principal stresses, the determination of σ′_(h) canbe performed eg by hydraulic fracture formation—more specifically bymeasuring the stress at which a hydraulically generated fracture doses.Determination of σ′_(H) can, in cases whenσ′_(V)+ξ(3σ′_(h)−σ′_(H))>3σ′_(h)−σ′_(H), where ξ express for theformation, for instance be performed by fracturing a vertical drilledwell, where the fracturing pressure will be a function of(σ′_(H)−σ′_(h)) and of σ′_(h). In cases when σ′_(v) is the larger of thethree principal stresses, the direction of σ′_(H) can be determined bymeasuring the orientation of a hydraulically generated fracture thatwill, provided the formation has isotropic strength properties, extendin a vertical plane coincident with σ′_(H). Prior knowledge of the valueof σ′_(H) is not essential if the invention is used to fracture wells ina well pattern that follows the direction of σ′_(H), as is preferred.

When production is performed in the field, liquids and/or gasses thatflow in the formation will change the state of stresses of theformation. For use in a continuous determination of the state ofstresses in the reservoir, in addition to knowledge of the initial stateof stresses, use may be made of a model calculation of the flow withinthe reservoir as well as a model calculation of the resulting effectivestresses in the reservoir rock. Flow simulation can be performed bystandard simulation software with measurements of production andinjection rates and pressures from the wells as input. From thecalculated stress field, the pressure gradient field can be derivedwhich determines the volume forces by which the solid formation isinfluenced in accordance with the following formula:b _(x) =−βdp/dx; b _(y) =−βdp/dy; b _(z) =−βdp/dz  1)wherein p is the pore pressure within the formation, while β is theBiot-factor of the formation and x, y and z are axes in a Carthesiansystem of co-ordinates. The effect of these volume forces on theeffective stress field in the formation will follow from the elasticitytheory and may be calculated eg by the method of finite elements.

By the reference numeral 2, FIG. 1 shows the course of the principalstress component σ′_(H) in the formation 1 in the shown plane followinga production period of six months. As seen, the orientation α of theeffective principal stress σ′_(H) relative to the drilled productionwells 5, 10 is relatively unaffected by the production a certaindistance from the production wells 5, 10. In the example, the angle αconstitutes about 25°. The designation γ further designates theorientation of σ′_(H) relative to a line indicated by the numeral 15that extends centrally between the drilled production wells 5, 10. Asseen, the angle γ corresponds approximately to the angle α in theexample shown.

It will also appear that the principal stress component σ′_(H)immediately at the drilled production wells 5, 10 has a modifiedorientation, the principal stress being oriented approximatelyperpendicular to the drilled production wells 5, 10, ie at an angle lessthan the angle β. In other words, the compressive stresses in theformation will, in this area, have a maximal component that is orientedapproximately perpendicular towards the drilled production wells 5, 10.This change of direction is initiated upon onset of production and isdue to the inflow in the drilled production wells 5, 10 of thesurrounding fluids.

FIG. 2 shows the development of the stresses σ′_(h) and the porepressure p in a cross sectional view through the formation in thesituation shown in FIG. 1 following a production period of six months,the lines 5′, 10′ indicating longitudinally extending vertical planesthat contain the drilled production wells 5, 10.

FIG. 3 shows how the method according to the invention can be exercisedwith the object of providing improved operating conditions from theproduction wells shown in FIG. 1 that will, in the following, bedesignated by the reference numerals 105, 110. The shown conditionscorrespond to the teachings shown with reference to FIG. 1 inasmuch asthe locations of the drilled production wells 105, 110 are concerned.

It will appear that, along a line corresponding to the line 15 of FIG.1, a further drilled well is produced that extends, in an area 125, fromthe formation to the surface where it is connected to a pump for thesupply of liquid, preferably sea water, to the drilled well section 115.The further drilled well section 115 will, in the following, bedesignated the ‘drilled injection well’.

Preferably the drilled injection well 115 has the same length as thedrilled production wells 105, 110 and will typically be unlined, meaningthat the wall of the drilled well is constituted by the porous materialof the formation 1 as such. However, the drilled well 115 can also belined.

Besides, FIG. 3 shows—by means of the curve family 102—the stressrelations in the formation 1 six months following the onset ofproduction. The stress relations reflect that, for a period of time T₁corresponding to the immediately preceding three months, liquid has beensupplied, preferably sea water or formation water, to the formation 1via the drilled injection well 115 and under particular pressureconditions that will be subject to a more detailed discussion below.

The supply of liquid to the porous formation generally involves—as wellknown—that the contents of oil or gas in the formation 1 between thedrilled production wells 105, 110 are, so to speak, displaced laterallytowards the drilled production wells 105, 110, whereby the fluidsinitially in place are produced more quickly. By the invention thesupplied liquid can be caused to give rise to further changes in thestate of stresses along the drilled injection well. As shown in FIG. 3,this can be verified by the angle γ′ between the line defined by thedrilled injection well 115 and the principal stress direction σ′_(H)being less than the corresponding angle γ for the conditions withoutsupply of liquid by the method according to the invention, see FIG. 1.This change is detected in the area along the entire drilled injectionwell. The fact that the orientation of σ′_(H) in the vicinity of theinjection well is oriented approximately in parallel with the drilledinjection well 115 contributes—as will be explained in further detailbelow—positively to achieving the effect intended by the invention. If,as is the case of a preferred embodiment of the invention, it isselected to form the drilled production wells 105, 110 and the drilledinjection well 115 such that, to the widest extent possible, they followthe orientation 102 of the natural effective principal stress σ′_(H) ofthe formation, it is possible to provide, at a very early stagefollowing the onset of liquid supply, advantageous conditions forachieving the effect intended with the invention.

As will appear from FIG. 4, which illustrates the state of stresses inthe formation 1 in the situation shown in FIG. 3, the value σ′_(h) inthe area at the drilled injection well 115 will, as a consequence of thesupplied liquid, be less than the corresponding value shown in FIG. 2.

As mentioned initially, the invention is based on the finding that,during the supply of liquid to a drilled injection well at elevatedinjection rates, undesirable fractures may occur that propagate from thedrilled injection well and into one of the adjoining drilled productionwells. Study of FIG. 3 will reveal such randomly extending fracture asoutlined by the reference numeral 200. The shown fracture extendsvertically out of the plane of the paper, but the fracture may—dependingon conditions prevailing in the formation 1—extend in any otherdirection.

By the invention it is aimed to benefit from the advantages that areassociated with a fracture that extends out of a drilled injection well.Study of FIG. 3 will show that by the invention it is, to a largeextent, possible to provide an advantageous fracture in the form of awidely vertical slot that extends along and coincides with the drilledinjection well 115.

In order to obtain the intended effect in accordance with the invention,liquid is initially supplied, while production is being carried out tothe drilled injection well 115 at a relatively low injection rate I.This state is maintained as a minimum for a period T₁ which will, asmentioned, cause the stress field to be reoriented around the drilledinjection well, whereby the numerically smallest normal stress componentσ′_(h) is oriented approximately perpendicular to the course of thedrilled injection well 115. In other words the smallest stress thatkeeps the formation under compression is oriented towards the plane inwhich it is desired to achieve the fracture. The liquid pressure P inthe drilled injection well 115 should, during the period T₁, be smallerthan or equal to the pressure P_(f), the fracturing pressure, thatcauses tension failure in the formation, and the injection rate I shall,during the period T₁, be smaller than or equal to the injection rateI_(max) that gives rise to tension failures in the formation.

Due to the supply of liquid to the drilled injection well 115, localstress changes will occur in the formation along the periphery of thedrilled injection well, and the invention makes use of this notch effectat the drilled well 115.

Above it was described how the flow of fluids changes the stress fieldin the reservoir. The resulting stress field can be calculated by addingthe stress changes to the initial state of stresses. In particular, thestresses can be evaluated along a line in the reservoir, position 115,along which an injector well has been drilled.

In the above the local variation of the stress field around thewells—caused by the occurrence of a hole in the formation—is notincluded. Within a radius from the drilled well of about three times theradius of the hole, the stress field will depend on the stress fieldevaluated along the line through the reservoir that the drilled wellfollows, but will differ significantly therefrom. The stresses on thesurface of the well bore as such are of particular interest to theinvention, in particular the smallest effective compressive stress—orthe largest tensile stress in case an actual state of tension occurs atthe hole wall. Such stress is in the following designated σ′_(hole,min).In cases where σ′_(hole,min) is a tensile stress, it is counted to benegative, whereas compressive stresses are always counted to bepositive. Calculation of σ′_(hole,min) presupposes in the following thatdeformations in the formation are linearly elastic. Given thiscondition, σ′_(hole,min) can be calculated by a person skilled in theart along a well track with any random orientation relative to anyrandom—but known—state of stresses.

In cases where a horizontal unlined injector is essentially parallelwith σ′_(H) (note that production and injection may cause thisparallelism, where it does not apply immediately at the time of drillingof the injector as indicated in FIG. 3), and where σ′_(V), σ′_(H),σ′_(h) are principal stresses calculated along the line in the reservoirwhere the well is drilled, and it further applies thatσ′_(V)>σ′_(H)>σ′_(h), σ′_(hole,min) is to be found on the top and bottomfaces of the hole and is given by the expression:σ′_(hole,min)=3σ′_(h)−σ′_(V)  2)wherein σ′_(h) and σ′_(v) are, in the present context, an expression ofthe effective stresses in the formation in the area of the position ofthe drilled injection well 115 determined on the basis of the elasticitytheory with due regard to the ingoing flows, cf. formula 1).

Also, in these cases around the drilled horizontal well, σ_(hole,min) isfound along the upper and lower parts of the drilled well, ie in tworegions that are in a horizontal plane as illustrated in FIG. 5. If thedrilled well 115 is circular, these areas are located where the verticaldiameter of the circle intersects the circle.

Since the liquid flow, as mentioned, gives rise to σ′_(h) decreasingover time, σ′_(hole,min) will decrease. It will appear from formula 2)that σ′_(hole,min, min) decreases when σ′_(v) increases. The productionfrom the drilled production wells 105, 110 gives rise to such increaseof σ′_(v).

In order to provide the desired fracture, the injection rate isincreased, as mentioned, after a certain period of time T₁ has elapsedsince the onset of the injection.

The condition that must be complied with to enable an increase in theinjection rate—and a controlled fracturing of the formation—is in allcases that the relationσ′_(hole,min)<σ′_(h)  3)has been complied with along the part of the well that is used forsteering the propagation of the fracture.

Provided the injection rate is increased prior to this condition beingcomplied with, ie before expiry of the requisite period of time T₁,there will be an increased risk of undesired fractures as describedabove.

The described course of events is illustrated in FIG. 6 that shows howthe injection of liquid is initiated about 90 days following onset ofproduction. At a point in time T₁ after onset of injection the aboverelation 3) has been complied with. In the example injection isperformed at the injection rate I for further 90 days, at which point intime σ′_(H) has advantageously undergone a considerable change oforientation (γ−γ′) of about 15°. Then the injection rate is increased toa value above I_(max), which is illustrated in FIG. 6 by the pressure inthe drilled injection well increasing. It will appear that σ′_(hole,min)abruptly changes character from compressive stress to tensile stress,whereby the tensile strength of the formation is reached, and fracturingresults.

It is noted that, in case the injection rate is not increased, accordingto the theory of the applicant, it is also possible to obtain, in thecase shown, the desired fracture when σ′_(hole,min), after a givenperiod, reaches the value of the tensile strength of the formation.However, in many cases this will cause substantial delays.

In FIG. 7 a typical measurement result is provided by the so-called‘step rate’ test for determining the maximally allowable injection rateI_(max). It is noted that in certain cases, it may be relevant toperform a continuous determination of the maximally allowable injectionrate I_(max). This is due to the fact that I_(max) may vary over time.Thus, during the period of time T₁ it may prove necessary to reduce theinjection rate I.

1. A method of controlling the direction of propagation of injectionfractures in a permeable formation (1), from which oil and/or gas isproduced, comprising: drilling in the formation (1), first and seconddrilled production wells (105, 110) next to each other; drilling at thedrilled production wells (105, 110), a further drilled well (115) thatextends between the first and the second drilled production wells (105,110); initiating production of oil and/or gas; while oil or gas is beingproduced, conveying a liquid to said further drilled well (115) and outinto the formation (1) for a first period of time T₁; performing atleast an approximated determination of the maximally allowable injectionrate I_(max) for the period T₁ in order to avoid producing fracturingruptures in said further drilled well (115) when liquid is suppliedtherein; keeping the injection rate I for the liquid supplied to thefurther drilled well (115) below said maximally allowable injection rateI_(max) for said time period T₁; and increasing the injection rate I toa value above I_(max) after expiry of the period of time T₁ when therelation σ′_(hole,min)<=σ′_(h) has been complied with along the furtherdrilled well (115), wherein σ′_(h) is the minimum horizontal effectivestress component and σ′_(hole,min) is the minimum effective compressivecircumferential stress at the wall of the further drilled well (115). 2.A method according to claim 1, comprising establishing the drilled wells(105,110,115) so as to have an essentially horizontal expanse.
 3. Amethod according to claim 1, comprising estimating prior toestablishment of the drilled wells (105, 110, 115) the direction (102)of the initial effective principal stress σ′_(h) of the formation in thearea of the planned location of the drilled wells, and the drilled wells(105, 110, 115) are formed so as to extend at an angle within +/−25°relative to this direction.
 4. A method according to claim 1, comprisingplacing the further drilled well (115) approximately equidistantlybetween the first and the second drilled wells (105, 110).
 5. A methodaccording to claim 1, comprising providing the further drilled well(115) with a lining prior to the supply of liquid.
 6. A method accordingto claim 1, comprising prior to said liquid being conveyed to thefurther drilled well (115), stimulating the further drilled well with aview to increasing the spreading of liquid in the formation.
 7. A methodaccording to claim 6 wherein the further drilled well is stimulated,prior to liquid being supplied thereto, by adding acid into the furtherdrilled well.